The present invention is primarily directed to cat cracker feed manufacture as used in fuels operations. More specifically, the present invention is directed at increased production of deasphalted oil and improved deasphalter operability in fuels processes.
Due to the decreasing quality of crudes and concomitant increase in the yield of residual oil fractions, residual oil upgrading processes have been included in many refineries. These upgrading conversion processes, which usually involve some form of catalytic treatment in a hydrogen atmosphere such as RESIDfining.TM. as described in Hydrocarbon Processing, September, p. 130 (1982), or resid visbreaking as described in Hydrocarbon Processing, January, p. 131 (1979) and Oil and Gas Journal, 79, (No. 15) p. 109 (1981), all three references hereby incorporated by reference, increase the yield of lighter products in both the distillation and deasphalting steps.
However, increased use of upgraded residua has resulted in the knowledge that the operation and capacity of a fuels deasphalting tower is substantially dependent upon the nature of the residua feed. Catalytic upgrading or thermal visbreaking of a residual oil prior to deasphalting results in large decreases in deasphalter capacity, efficiency, and considerable loss in deasphalted oil yield.
The principal cause of this situation is the result of formation of a three-phase system, in which the third phase is usually an immiscible solid asphaltic mixture, which is incompatible with both the deasphalted oil extract and the asphaltic raffinate phases. Thus, any modification to a deasphalting process which tends to reduce or eliminate formation of this third phase when using upgraded residua would simultaneously increase yield and capacity and would represent a major benefit to a refiner.
U.S. Pat. Nos. 2,700,637, 2,934,715 and 2,882,219 disclose the addition of cycle oil or decant oil (Cat Fractionator Bottoms), respectively, to a deasphalter feed in order to increase the yield and quality of suitable catalytic cracking feedstock. In each of these cases, the deasphalter feed constitutes a conventional vacuum residual oil which has not been catalytically or thermally pretreated, which has been found to be the necessary condition for immiscibility via third phase formation in a fuels deasphalting operation. None of these patents include any teaching for overcoming this third phase formation and providing improved deasphalter and feed heat exchanger operability, specifically utilizing an upgraded vacuum residuum.
U.S. Pat. No. 2,570,044 discloses recycling of an aromatic extract oil stream derived from a deasphalted oil to a deasphalter feed during lubes production. This is disclosed as being carried out in order to eliminate the formation of a third phase which tends to foul the deasphalter internals. However, the deasphalter feed cited in U.S. Pat. No. 2,570,044 has not been subjected to an upgrading process. Further, the nature of the third phase must be considerably dissimilar to that discussed in the present invention since addition of an aromatic extract oil derived from a deasphalted oil is not successful in the suppression of third phase formation during the deasphalting of an upgraded residuum.
What is desired in the art is a process in which the overall production of deasphalted oil is maximized while maintaining specification quality for fuels manufacture derived from vacuum distilled residua that have been catalytically treated or thermally treated in a residuum upgrading step. The specifications involve maintaining a low Conradson Carbon Residue and low metals content, so that valuable fuels end products can be produced by further processing.